Leeds U.S. Pat. Nos 2,777,824 and 3,055,297 disclose marking structures made of highly porous plastic material, the pores of which are of microscopic proportions and are filled with a marking fluid such as an ink. Structures made in accordance with the teachings of the Leeds patents have experienced a high degree of commercial success as hand stamps, stamp pads, and also as ink rolls such as are used for applying ink to printing members in automatic printing equipment. Such structures are advantageous because of their long life, both in length of time and in numbers of operations or impressions, and because they operate well without the necessity of repetitive re-inking of the marking surface. As ink rolls sold under the trademark Porelon.RTM., such structures provide uniform inking of printing members and exhibit fast recovery to facilitate repetitive use over long periods of time.
A variety of different ink roll structures, microporous foam materials and processes for making such materials are known. Some, for example, feature a bilayered approach wherein one of the layers acts as an ink reservoir. See Hansen U.S. Pat. No. 3,971,315, issued Jul. 27, 1976 and Fujimura U.S. Pat. No. 4,306,498, issued December, 1981. Copolymers of polyvinyl acetate and polyvinyl chloride (PVC) have been used as an ink impregnated material for use in hand stamps. See Ooms et al. U.S. Pat. No. 4,927,695, issued May 22, 1990. Ink rolls have been prepared by a variety of different methods, including a salt leaching process (Piepmeier, Jr., et al. U.S. Pat. No. 4,768,437, issued Sep. 6, 1988) and explosion reticulation (Meisel, Jr. et al. U.S. Pat. No. 3,297,803, issued Jan. 10, 1967.)
Conventional printing systems have difficulty with sequential numbering and imprinting operations wherein a limited area of a printed signature must be printed with a unique number or message, such as the code number of a coupon. Microporous ink rolls have not been used in high speed printing presses because the high rotational speeds involved cause the ink roll to "mist", i e., centrifugal forces cause the ink roll to release droplets of the ink into the air. Known ink rolls also lack sufficient strength and other physical characteristics to perform at high speeds for extended periods. The present invention addresses these problems by providing an improved microporous ink roll capable of performing at high rotational speeds.